Granular detergent compositions have so far been principally prepared by spray drying. In the spray drying process the detergent components, such as surfactants and builders, are mixed with as much as 35-50% water to form a slurry. The slurry obtained is heated and spray dried, which is expensive. A good agglomeration process, however, could be less expensive.
Spray drying requires 30-40 wt. % of the water to be removed. The equipment used to produce spray dry is expensive. The granule obtained has good solubility but a low bulk density, so the packing volume is large. Also, the flow properties of the granule obtained by spray drying are adversely affected by large surface irregularities, and thus the granulate has a poor appearance. There are other known disadvantages in preparing granular detergents by spray drying.
There are many prior art nonspray-drying processes which produce detergent granules. They have drawbacks as well. Most require more than one mixer and a separate granulation operation. Others require use of the acid form of the surfactant to work. Some others require high temperatures which degrade the starting materials. High active surfactant paste is avoided in these processes because of its stickiness.
EP-A-0 345 090, published Dec. 6, 1989, discloses a process for manufacturing particulate detergent compositions comprising contacting detergent acid with neutralizing agents and providing particulates by contacting the detergent acid with a particulate neutralizing agent or detergent salt with carrier in an absorption zone.
EP-A-0 349 201, published Jan. 3, 1990, discloses a process for preparing condensed detergent granules by finely dispersing dry detergent builders and a high active surfactant put into a uniform dough which is subsequently chilled and granulated using fine dispersion to form uniform, free flowing granular particles.
EP-0 390 251, published Oct. 3, 1990, discloses a process for the continuous preparation of a granular detergent or composition comprising steps of treating, firstly, particulate starting material of detergent surfactant and builders in a high-spead mixer, secondly in a moderate-speed granulator/densifier and thirdly in a drying/cooling apparatus, with the addition of powder in the second or between the first and second step to reduce the amount of oversize particles.
A. Davidsohn and B. M. Mildwidsky, Synthetic Detergents, John Wiley & Sons 6th edition, 1978, discloses general detergency teachings, including the manufacturing of finished detergent products.
High shear and cold mixing processes per se are known, but they require an extra grinding step or some other action. E.g., some use a dry neutralization technique of mixing an acid form of the surfactant with sodium carbonate. See U.S. Pat. No. 4,515,707, Brooks, issued May 7, 1985; Japanese laid-open Appln. No. 183540/1983, Kao Soap Co., Ltd., filed Sept. 30, 1983; and Japanese Sho. 61-118500, Lion K. K., Jun. 5, 1986. Typically, excess carbonate is required (2-20 molar excess) to assure reasonable conversion of the surfactant acids. Excess carbonate adversely drives up the wash water pH to the very alkaline range which can be undesirable, particularly for some low-phosphate formulas.
The use of a surfactant acid generally requires immediate use or cool temperature storage, for highly reactive acids such as the alkyl sulfate acids are subject to degradation unless cooled, they tend to undergo hydrolysis during storage, forming free sulfuric acid and alcohol. In practical terms, such prior art processes require close-coupling of surfactant acid production with granulation which requires an additional capital investment.
A second route, well known in the field and described in the patent literature, is the in-situ neutralisation of the anionic surfactant acid with caustic solutions (e.g. 50% NaOH) or caustic powders (e.g. Na.sub.2 CO.sub.3) right before or in the course of the granulation step. In this situation, precautions are needed to ensure complete neutralisation of the acid to avoid undesirable effects on the rest of the surfactant matrix upon storage/or during the wash. The resulting particle is a highly dense granule which can be incorporated into granular detergents.
While this second route uses lower temperatures and less drastic shear conditions than crutching and spray drying, it has many limitations. On one side the need to carry out a chemical reaction (neutralization) during or right before the granulation step limits considerably the range of processing conditions that can be used (temperature, chemicals, etc.). The very low pH of the anionic surfactant acid prevents the incorporation of chemicals sensitive to these acidic conditions. But above all, in the case of those anionic surfactants which are not chemically stable in the acid form or physically unstable, this process requires the close coupling of the sulphation/sulphonation unit with the neutralization/granulation step. This results in considerable limitations in the logistics and/or the design of the facilities for these processes as well as an important increase in the complexity and difficulty of the control systems for the overall process.
The present invention brings solutions to the problems mentioned above and provides with a more flexible and versatile route to the processing of granular detergents. The present invention is based on an agglomeration/granulation step that is completely uncoupled from the sulphation/sulphonation process. The basis of the invention is the introduction of the anionic surfactant in an aqueous, highly concentrated solution of its salt, most preferably of its sodium salt. These high active (low moisture) surfactant pastes are of a high viscosity but remain pumpable at temperatures at which the surfactants are stable. This guarantees the ability to transport and transfer the chemical from the manufacturing location to the granulation site and to be able to have adequate storage facilities prior to the formation of a particle. For those cases where both the sulphation/sulphonation is already immediately preceding the granulation step, it provides the possibility to install intermediate buffer tanks that simplifies the control of the whole unit. In the case of some anionic surfactants or mixtures of them where highly viscous liquid crystal phases occur, this technology requires that either lower viscous phases can be formed (e.g. neat phases) or that some viscosity modifiers are used (e.g. hydrotropes).
An important object of the present invention is to make a dense, concentrated detergent granular product by an agglomeration process as opposed to a spray-drying process. It is another object of the invention to provide for a granular detergent product having a good solubility and good dispersion properties, and improved dispensing from a washing machine.
Other objects of the present invention will be apparent in view of the following.